Liquid crystal displays are widely used in personal computers, mobile equipment monitors and televisions since they have various advantages, e.g., in their low voltage and low consumption power and high possibility for reduction in size and profile. Although a variety of modes depending on how liquid crystalline molecules are aligned in a liquid crystal cell have been proposed for such liquid crystal displays, the dominating mode has hitherto been a TN mode in which liquid crystalline molecules are in an aligned state that their orientations twist by about 90° toward an upper side substrate from a lower side substrate.
In general a liquid crystal display is made up of a liquid crystal cell, an optical compensation film and a polarizer. The optical compensation film is used for dissolution of coloring of images and expansion of a viewing angle, and a stretched birefringent film or a transparent film coated with a liquid crystal is employed as the optical compensation film. For instance, Japanese Patent No. 2587398 discloses the art of expanding a viewing angle by applying to a TN-mode liquid crystal cell the optical compensation film formed by coating a discotic liquid crystal on a triacetyl cellulose film, forcing the liquid crystal into an aligned state and fixing the aligned state. However, liquid crystal displays for television use, which are supposed to be equipped with big screens and to be viewed from various angles, have stringent demands on viewing angle dependence, so even the foregoing art cannot satisfy such demands. Under these circumstances, liquid crystal displays employing modes different from the TN mode, such as an IPS (In-Plane Switching) mode, an OCB (Optically Compensatory Bend) mode and a VA (Vertically Aligned) mode, have been studied. Since the OCB mode in particular offers a high response speed and has a high suitability for moving images, attention is being given to this mode as an operation mode of liquid crystal displays for TV use.
Cellulose acylate films have a feature that they are high in optical isotropy (low in retardation value), compared with other polymer films. Accordingly, it is a general rule that cellulose acetate film is used for applications requiring optical isotropy, such as for polarizing plates.
By contrast, optical anisotropy (high retardation value) is required of optical compensation films (retardation films) used in liquid crystal displays. The optical compensation films for OCB-mode in particular are required to have an in-plane retardation (Re) of 20 nm to 100 nm and a thickness-direction retardation (Rth) of 70 nm to 400 nm.
Hitherto, it has been a general rule in the technical field of optical materials that synthetic polymer films are used in the case of requiring for polymer films to have optical anisotropy (high retardation values Re and Rth), whereas cellulose acetate film is used in the case of requiring for polymer films to have optical isotropy (low retardation values).
EP-A-911656 discloses the cellulose acetate film having high retardation values which, though against the rule hitherto regarded as general, is also usable for applications requiring optical anisotropy. In EP-A-911656, a compound having at least two aromatic rings, notably a compound having a 1,3,5-triazine ring, is added and stretch processing is performed in order to achieve high retardation values in the case of using cellulose acetate.
Although it is generally known that cellulose acetate is a polymer material hard to stretch and its birefringence factor is difficult to increase, the patent document cited makes it possible to increase the birefringence factor through simultaneous alignment of the additive molecules by stretch processing and achieves high retardation values. Such a film can also serve as protective film of a polarizing plate, so it has an advantage in its suitability for offering thin liquid crystal displays at low prices.
JP-A-2002-71957 discloses the optical film which contains a cellulose ester having acyl groups having 2-4 carbon atoms as substituents and satisfying both the relations 2.0≦A+B≦3.0 and A<2.4 when the degree of acetyl-group substitution is taken as A and the degree of propionyl- or butyryl-group substitution is taken as B, and further satisfying the relation 0.0005≦Nx−Ny≦0.0050 when the refractive index in the direction of a slow axis and the refractive index in the direction of a fast axis at the wavelength of 590 nm are taken as Nx and Ny, respectively.
JP-A-9-211444 and JP-A-11-316378 achieve wide viewing angles in the polarizing plates used for OCB-mode liquid crystal displays by application of optical compensation films having layers made from liquid crystalline compounds.
The methods disclosed in the documents cited above are effective in allowing low-profile liquid crystal displays to be produced at low prices. In recent years, however, there has been a growth in the use of liquid crystal displays under a wide variety of circumstances, and it has become a problem that the cellulose ester films utilizing the foregoing arts changed their optical compensation performance under those circumstances. More specifically, the problem is in that those cellulose ester films are affected by changes of circumstances, notably humidity, especially when each film is stacked on a liquid crystal cell, and they change their Re and Rth retardation values to result in changes of their optical compensation capabilities. Therefore, it has been requested to solve such a problem.
With the upsizing of liquid crystal displays, on the other hand, there has been a growing need for improvements in light leakage occurring from the periphery of the display screen by change in circumstances, which has become a big problem in addition to the circumstances-causing changes in optical properties. Descriptions about such light leakage can be found in JP-A-2002-139621 and JP-A-2002-169023.